It is known that the balloon equilibrium at a given altitude in the atmosphere depends on the displaced volume of air of which the mass must be balanced by the sum of the solid and gaseous masses carried by the balloon. Stable equilibrium is achieved at a given altitude when the vertical balloon displacement takes place in the presence of at least one parameter being modified (which concerns the balloon or the ambient atmosphere), tending to return the balloon to its starting point.
Thus, only a pressurized balloon within which the pressurized gas always remains at constant volume is inherently stable at a given altitude. A balloon which is open at its lower part is stable in ascent due to the natural evacuation of the aerostatical gas that takes place at constant volume; but on the other hand it is unstable in descent when its volume decreases. A balloon far from its filled state (i.e., when limp) is unstable and rises or descends depending on the thermal balance controlling its gas-bulb volume and the change in the gas mass (such a change being caused by diverse phenomena and in particular being due to diffusion through the envelope.
Now it is often desired in many missions (particularly scientific missions) to keep a balloon for some time at a constant altitude or only slowly varying its altitude in ascent or descent at a given altitude or at several desired altitude levels. Frequently the altitude levels that are desired correspond to a limp balloon state.
To assure this stabilization (where this term means both staying at a constant altitude and rising or descending in controlled manner), the generally adopted procedure is to modify the specific weight of the balloon by either jettisoning ballast or evacuating part of its aerostatic gas. These operations are initiated by electromechanical systems which in some instances are ground-controlled by a remote radio-electric control, and in other cases from the balloon by pick-ups mounted to it which measure certain parameters, typically the temperature and the atmospheric pressure, which after analysis allow effecting the controls. However the ambient atmosphere as a rule will not be calm due to vertical winds, turbulence etc., and it is a fairly complex matter to collect useful parameters. The corresponding equipment, which must be functional through the entire balloon lift and sometimes in a harsh environment, strongly deviates from the ideal rated operation of the electronic and supply circuits (temperatures less than -100.degree. C. or exceeding +100.degree. C.), and such equipment is complex, costly and heavy. Moreover, it generally uses up a lot of ballast and accordingly is poorly suited for the atmospheric-planet type of exploration wherein the total balloon mass must be as low as possible (the balloon is understood to be the balloon proper and all material therein).
The object of the present invention is thus to overcome the drawbacks of the conventional systems and to provide an altitude-stabilizing system for a balloon. The invention, deals with a process for stabilizing an atmospheric balloon of the type comprising means to vary its specific weight, in particular jettisoning means designed to reduce the balloon mass, means for evacuating the aerostatic gas, or, in the case of a hot-air balloon, a burner fed by a fluid fuel.
A particular object of the invention is to assure balloon-stabilization by using substantially simplified equipment of which the cost and the weight are significantly reduced with respect to those of conventional systems.
Another object is to create novel balloon forms which are better suited to missions which include predetermined-altitude stabilizations.
To that end the process of the invention for altitude-stabilizing an atmospheric balloon and comprising means to vary its specific weight comprises the use of a balloon provided with an inter-pole link extending from its upper to its lower poles and in controlling means designed to vary the balloon's specific weight using this inter-pole link in such a manner that an increase in the tension T applied to said link shall act on these means to reduce the specific weight.
The term "inter-pole link" denotes any component or assembly of components of longitudinal shape extending between the poles of the balloon and physically fastened either to the poles themselves or near them, or to auxiliary manner, this link as a rule will be flexible and can be one cable, several cables, one flexible sleeve or several sleeves, etc.
The balloon being used can be of the type described in the French patent application 80.00343 filed by the applicant on Jan. 4, 1980 (or U.S. Pat. No. 4,420,130) and evincing an envelope with symmetry of revolution about one axis and comprising an approximately cylindrical portion.
The length L of the inter-pole link is adjusted as a function of the contemplated mission to a value depending on the natural distance D.sub.p between the two poles when the balloon is filled. To keep the balloon limp at a constant altitude or at a slowly varying altitude, this length L generally shall be adjusted to a value L.sub.o .gtoreq.D.sub.p.
When the balloon in its filled state is descending, the distance D between the poles increases from the value D.sub.p. In the absence of the inter-pole link, this distance might grow to a limiting value D.sub.F very close to the envelope gore length. In the presence of the inter-pole link, which exceeds D.sub.p but is less than D.sub.F, when the inter-pole distance D becomes equal to the length L.sub.o of the link, a tension T arises in the link which increases as the descent continues and can decrease only if the balloon rises again.
According to the invention this tension T is used to actuate means for varying the balloon's specific weight: when this tension exceeds a threshold, it acts on these means to reduce the specific weight, whereby the descent is decelerated and the balloon is stabilized.
In the case of a balloon equipped with jettison means known per se, these means are controlled by the inter-pole link so as to cause jettison when the tension T exceeds a predetermined tension threshold t.sub.d.
In the case of a hot-air balloon equipped with suitable combustion-fluid burners, the combustion rate is controlled by the inter-pole link in such a manner that said rate increases when the tension T exceeds a given threshold t.sub.d '.
For some missions and some balloon types, the descending stabilization implemented by the above defined process will be enough (considering in particular the natural ascent stability of certain balloons, as already mentioned).
In other cases however a balloon is used which comprises means for varying its specific weight both by lowering and increasing it. In the process of the invention, these means are controlled by the inter-pole link so that an increase in the tension T in said link acts on these means to reduce the specific weight and a decrease in said tension acts to increase said specific weight.
With respect to an atmospheric balloon comprising both jettison means and aerostatic gas evacuation means, the link controls the jettison when its tension exceeds the threshold t.sub.d and controls the evacuation when this tension drops below a threshold t.sub.e &lt;t.sub.d. To stabilize the balloon at a desired altitude level, it suffices to adjust the length L of the inter-pole link and the thresholds t.sub.e and T.sub.d so that the tension T in the link at the altitude of the desired level be between t.sub.e and t.sub.d. The link is inactive as long as its tension remains between these values (that is, as long as the balloon remains at the desired altitude level). If, for some reason, the balloon should begin the descend, the tension T increases and beyond the threshold t.sub.d causes a jettison whereby the descent is stopped and a new ascent takes place. If on the other hand the balloon should rise, the tension T drops, and below the threshold t.sub.e causes evacuation of the aerostatic gas, whereby the rise stops and a new descent takes place. It must be borne in mind that the condition t.sub.e &lt;t.sub.d prevents simultaneous occurrence of jettison and aerostatic gas evacuation.
In the case of a hot-air ballon, the combustion-gas flow-rate is controlled by the inter-pole link so as to increase from an equilibrium value when the tension T in the link rises above the threshold t.sub.d ' and to decrease from this equilibrium value when the tension T drops below a threshold t.sub.e '.ltoreq.t.sub.d '.
The present invention also covers an atmospheric balloon to implement the process described above. This balloon comprises an envelope made of a flexible hermetic material, a jettison reservoir located below the balloon, a jettison member associated with the reservoir to cause a controlled jettison of ballast, and, possibly, an aerostatic gas evacuation aperture in the ballon envelope with a movable sealing member for this aperture. In the present invention, said balloon is provided with an inter-pole link between its upper and lower poles, said link being fastened to the jettison member so as to act on it beyond a predetermined tension-threshold t.sub.d ' and where such is present, also fastened to the movable sealing member so as to close it beyond a predetermined tension threshold t.sub.e.
The invention furthermore covers a hot-air aerostatic balloon comprising an envelope made of a flexible, hermetic material and open at its lower part, a burner hooked-up in the vicinity of this lower pole, and means for supplying the burner with combustion fluid from a tank. According to the invention, the aerostatic balloon comprises an inter-pole link between the upper and lower poles of the balloon, the link being fastened to the burner supply means so as to adjust the combustion fluid flow rate as a direct function of the tension in said link.